The present invention relates to the field of harp screens and provides solutions to a number of long recognized but unsolved shortcomings associated with harp screens.
Harp screens have long been known to the art and recognized as efficient devices for separating and classifying fragmented material. Typically, such harp screens have a rectangular screen box frame and utilize a pluarality of individually mounted and tensioned wires which extend longitudinally between the ends of the frame. Generally a plurality of transverse beams extend across the screen box frame, and grooves are cut into the beams so as to receive the longitudinal wires and maintain a predetermined distance of separation between the wires. Commonly, each wire is fixed to the ends of the screen box frame and tensioned by a screw device similar to that used for the tensioning of piano wires.
Such harp screens are effective classification devices but are expensive to manufacture and maintain. For example, each of the transverse supporting beams must have a multiplicity of equally spaced grooves to receive the longitudinally wires and such grooves must be individually cut during manufacturing. No matter how much care is used in the selection of the beams, the grooves tned to wear down in constant use and eventually the grooves will no longer adequately contain the wires, which then slip laterally, changing the separation distance between wires and ruining the accuracy of the screen as a sorting device. Once the grooves in the transverse beams have become worn and unreliable, the entire beam must be removed and replaced. Since the beams are welded to the side walls of the frame, such removal and replacement can be time consuming, difficult, and expensive. Aside from the expense and inconvenience of replacing worn beams, the use of beams with such precut spaced apart grooves practically destroys any versatility of the screen box since the spacing of the wires of the screen cannot be varied once the cuts have been made, and hence the coarseness of the screen is determined with finality.
There have been attempts to reduce the effort needed to replace worn beams, and one of the more popular solutions has been to substitute threaded, hollow rods or pipes for the transverse beams with the individual thread cuts service as grooves in which to contain the longitudinal wires. While this arrangement provides fairly satisfactory initial spacing between wires, the thread cuts are inherently weak at their peaks and in the heavy wear situations encountered by harp screens, the threads wear down rapidly and fail. Soon screen wires start slipping from their initial thread slot positions, move to adjacent slots, are captured in the adjacent slots and do not normally return to the initial slot. The result is that the shortened wear life of threaded rods leads to irregular wire spacing and inacceptable inaccuracies in sorting of fragmented material.
Still another shortcoming of the use of threaded rods or pipes is that the spacing of wires is limited to the pitch of the thread rods or to multiples thereof. In addition, in order to align the adjacent wires passing over successive threaded rods so as to obtain a straight line path for each wire, the rods must commonly be rotated and manipulated so as to properly align the threads. Such manipulation and rotation is difficult and time consuming and must be carefully done before the threaded rods are welded or fixed to the side walls of the screen box. Even after alignment is achieved, the positioning of the wires in the thread cuts is time consuming and slow because the workman must position the yet untightened wires and count the number of thread cuts between adjacent wires so as to obtain the desired spacing, and it has been found that spacing errors are hard to avoid when all thread cuts are not used and all wires are not at final levels of tension.
Finally, the threaded rods are seldom formed of solid material throughout, and normally the rods must be hollow in order to avoid adding so much additional weight and mass to the screen box that the shaking of the screen box becomes overly difficult. Aside from the weight problem, a solid rod would be preferred because it is more likely to be relatively straight and regular in its diameter. It has been found that hollow threaded rods are seldom perfectly straight nor are they constant in their diameter and these irregularities can effect wire spacing and screen accuracy. Additionally, the hollow rods tend to vibrate more than solid rods and this can lead to dislodgement of the wires with resulting changes in spacing, higher wear, and reduced accuracy.
Another shortcoming associated with the use of threaded rods or pipes is that the thread cuts are nonparallel to the wires which define the screen. This nonparallelism results in an almost saw-like action between each thread cut and wire during vibration and this increased friction reduces the use life of both the threaded rods and the wires.
With all known harp screen constructions, the tensioning and retensioning of individual wires is extremely time consuming and slow because the tensioning devices now used on such screens do not adapt readily to conventional torque wrenches.
Accordingly, it is desirable that all the described shortcomings be reduced or alleviated in order that harp screens require less labor to manufacture and maintain, that the desired spacing of wires can be retained for longer intervals and that repair or downtime be reduced. The present invention provides a workable and commercially feasible solution to these long recognized but unsolved problems.